It means using wireless communications over multiple hops, for example wireless sensor networks. Imagine deploying Arduino-based nodes for monitoring environmental conditions and sensor data has to be relayed to a base station over several wireless hops.

About 20USD for the stamp. There is a link to the e-shop on the site. Fairly simple, and shipping is fast.Plenty of "carrier boards" as well, I used battery and relay ones, not really cheap, but work great.

The RF22 is half the price of the Panstamp, but it does not include the Arduino chip. This makes the Panstamp cheaper if you need one anyway. What is the expected range of the Panstamp in an urban environment? The RF22 is about 150m.